Tectonophysics 392 (2004) 193 – 218
www.elsevier.com/locate/tecto
Tectonic mechanisms associated with P–T paths of regional
metamorphism: alternatives to single-cycle thrusting and heating
John Wakabayashi *
Wakabayashi Geologic Consulting, 1329 Sheridan Lane, Hayward, CA 94544, USA
Available online 24 June 2004
Abstract
Metamorphic pressure ( P) – temperature (T) paths are commonly used as tools to interpret the tectonic history of orogenic
belts, those deformed belts of rocks that record past activity along active plate margins. Many studies and reviews relating P – T
path development to tectonics have focused on thrusting – thermal relaxation cycles, with special emphasis on collisional
processes. Other studies have assumed that P – T paths resulted from a single tectono-metamorphic event that accounted for the
entire burial – exhumation history of the rocks. In many cases, such assumptions may prove invalid.
This paper speculates on the relationship of tectonic processes other than thrusting – heating to P – T path development. The
processes discussed herein include subduction initiation, triple-junction interactions, initiation and shut off of arc volcanism,
subcontinental delamination, and hot spot migration. All of these processes may leave a signature in the metamorphic rock
record. Examples are presented from a number of localities, most of which are from the Pacific Rim. Although thrusting –
heating cycles have influenced metamorphic evolution in many orogenic belts, the potential impact of other types of tectonic
mechanisms should not be overlooked.
D 2004 Elsevier B.V. All rights reserved.
Keywords: P – T paths; Tectonics; Orogenic belts; Subduction initiation; Triple-junction migration; Magmatic arcs
1. Introduction
The study of ancient, exhumed continental margins
commonly focuses on deformed belts of rocks known
as orogenic belts, because these regions formed as a
consequence of active plate margin tectonics (e.g.,
Dewey and Bird, 1970; Moores, 1970). Within orogenic belts, geoscientists use the spatial distribution of
different rock types, their ages, and their structural and
metamorphic history, to better understand the tectonics of ancient plate boundaries.
* Fax: +1-510-887-2389.
E-mail address: wako@tdl.com (J. Wakabayashi).
0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2004.04.012
Studies linking tectonic environments to types of
metamorphic rocks, with key examples from the
Pacific Rim and Alpine regions, were published as
plate tectonic theory became widely accepted (e.g.,
Miyashiro, 1967, 1973; Ernst, 1971). Oxburgh and
Turcotte (1974) conducted pioneering thermal modeling of pressure ( P)– temperature (T)– time (t) paths
that represent the progressive metamorphic evolution
of rocks. Oxburgh and Turcotte (1974) based the
thermal models on a process that involved thrusting,
that buried rocks faster than they can equilibrate with
the ambient geothermal gradient, followed by thermal relaxation, which reestablished the ambient geothermal gradient in underthrust rocks. The thrusting –
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
thermal relaxation cycle was linked to subduction
followed by collision. Subsequent researchers refined
thermal models based on thrusting –thermal relaxation cycles and evaluated clockwise ( P on the
positive y axis) P– T paths of orogenic belts in this
context (e.g., England and Thompson, 1984; Thompson and England, 1984; Thompson and Ridley, 1987;
Spear and Peacock, 1989) (Fig. 1G). Since the late
1980s, an increasing number of studies have interpreted P – T paths that differ significantly from
clockwise P – T loops (Fig. 1, Table 1). These studies
include those of some types of granulites (e.g.,
Bohlen, 1987; Harley, 1989) (Fig. 1C), low-P/highT metamorphism (e.g., Clarke et al., 1987; Grambling, 1988; Rubenach, 1992; Sisson and Pavlis,
1993; Johnson and Vernon, 1995; Brown, 1998a,b)
(Fig. 1B), subduction – transform transition (Wakabayashi, 1996) (Fig. 1D, H), and of rocks associated
subduction initiation (e.g., Wakabayashi, 1990;
Hacker, 1991; Parkinson, 1996; Hacker and Gnos,
1997) (Fig. 1A).
This paper focuses on evaluation of P – T paths
formed by mechanisms other than thrusting – thermal
relaxation. The mechanisms reviewed in this paper are
overlooked in many analyses of P –T paths. Fig. 1
shows examples of some of the types of P –T paths
that I will discuss in the paper. These and other P – T
paths are listed in tabular form in Table 1. The first
sections of the paper discuss some general definitions
and concepts related to tectonic interpretations of P – T
paths. Following the general sections, I will speculate
on specific tectonic settings of P –T path development. Most of the examples used in these discussions
are from the Pacific Rim region, with particular
emphasis on western North America. This paper is
not intended as a comprehensive review of P – T paths
and tectonics. My goal is merely to point out that P – T
paths we interpret from rocks are likely generated by a
greater variety of tectonic processes than commonly
proposed.
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2. Partial P – T paths and ‘apparent’ P – T paths
2.1. Partial versus complete P –T loops
Mineral growth recording a P – T path will not
reflect the complete burial and exhumation history
of a rock. This is because metamorphic minerals do
not grow during the lowest grade parts of the burial
and exhumation paths, and because successive recrystallization may erase earlier formed assemblages.
Collisional thrusting – exhumation, and subduction
with syn-subduction exhumation can account for an
entire burial–exhumation cycle of a rock (Fig. 2A to
J). Basinal sedimentation followed by basin inversion
may also result in a complete burial – exhumation
cycle, but the maximum depths of burial are much
less than in collisional and subduction settings (e.g.,
Cooper and Williams, 1989). For other tectonic mechanisms, the rocks that record P – T paths associated
with the mechanism may be at significant depth when
the tectonothermal event begins and ends (Fig. 2K –
U). Multiple tectonic events must occur in order to
complete a burial–exhumation cycle for such rocks. If
there is no metamorphic record of exhumation and
initial burial, however, the P –T path recorded in such
rocks may be the product of a single metamorphic
event (Fig. 2K – U).
2.2. Time dependence and ‘apparent’ P –T paths
P – T paths recorded by overprinting mineral
assemblages, inclusions, and mineral zoning are
generally interpreted to result from a single evolving
tectono-metamorphic event (e.g., Oxburgh and Turcotte, 1974; England and Thompson, 1984; Thompson and England, 1984; Ernst, 1988). However, the
different stages of metamorphic mineral growth in
rocks can result from metamorphic events separated
by long periods of time and related to different
tectonic events (e.g., Getty et al., 1993; Hand et al.,
Fig. 1. Examples of different P – T path types. Most examples are P – T paths determined from field-based studies. Modeled P – T paths are
denoted by an asterisk after the acronym. Labels for P – T paths correspond to the following references: A02*: Aoya et al. (2002); B87: Bohlen
(1987); B98: Baba (1998); H91*: Hacker (1991); HD92: Henry and Dokka (1992); HG97: Hacker and Gnos (1997); M85: Maruyama et al.
(1985); ML88: Maruyama and Liou (1988); JV95: Johnson and Vernon (1995); OB90: Otsuki and Banno (1990); R92: Ring (1992); Rb92:
Rubenach (1992); S89: Sisson et al. (1989); S99: Smith et al. (1999); SP89; Stüwe and Powell (1989); SP99: Soto and Platt (1999); TR87*:
Thompson and Ridley (1987); W90: Wakabayashi (1990); W96*: Wakabayashi (1996); Y82: Yardley (1982); P – T paths in ‘‘D’’ from
Wakabayashi (1996). Facies abbreviations are Am: Amphibolite, Bs: Blueschist, EA: Epidote amphibolite, Ec: Eclogite, Gr: Granulite, Gs:
Greenschist, PP/Z: Prehnite – pumpellyite and Zeolite. Facies boundaries from Takasu (1984).
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
Table 1
Examples of P – T paths generated by processes other than
thrusting – thermal relaxationa
P – T path type
Location
Reference
Anticlockwise:
inception of
subduction
High-grade blocks,
Franciscan
Complex,
California, USA
Eclogites, Yukon –
Tanana terrane,
Canada
Structurally highest
part of Shuksan
Suite, Washington,
USA
Raspberry Schist,
Alaska, USA
Tetagouche Group,
New Brunswick,
Canada
Structurally highest
unit of Villa de
Cura belt,
Venezuela
High-grade blocks,
Coastal Chile
Anglesey
blueschists, Wales
Early stages of
metamorphism,
structurally highest
part of Sanbagawa
Belt, Japan
Kamuikotan zone,
Hokkaido, Japan
Wakabayashi (1990),
Krogh et al. (1994)
Ballantrae
Complex, Scotland
Metamorphic sole
of Oman ophiolite
Metamorphic sole
beneath Kiziltepe
ophiolitic nappe,
Turkey
Metamorphic sole,
Kaynarca area,
Tavsanli Zone,
Turkey
High grade blocks,
Sulawesi
Fragments from
Marianas
serpentinite
seamounts
Perchuk et al.
(1999)
Brown et al. (1982)
(y)
Table 1 (continued)
P – T path type
Location
Reference
Anticlockwise:
inception of
subduction
Anticlockwise
(acw) or
isobaric
cooling (ibc),
medium to
high P
granulites
Platta nappe, upper
Engadine Valley,
Switzerland Alps.
Granulites, many
locations in listed
references (mostly
ibc, some acw)
Kohistan Arc,
Pakistan (acw)
Lewisian Complex,
Outer Hebrides,
Scotland (acw)
Early
metamorphism,
Mojave extensional
belt, California,
USA (acw)
Chugach Complex,
Alaska, USA (ibh)
Phillip (1982) (y),
Deutsch (1983) (y)
Roeske (1986) (y)
Trzcienski et al.
(1984), van Staal
et al. (1990) (y)
Smith et al. (1999)
Kato and Godoy
(1995)
Gibbons and Gyopari
(1986)
Hara et al. (1990)
(y); Wintsch et al.
(1999)
Ishizuka et al.
(1983) (y);
Maekawa (1986) (y)
Bloxam and Allen
(1960) (y)
Hacker and Gnos
(1997)
Dilek and Whitney
(1997)
Anticlockwise
(acw), isobaric
cooling (ibc),
isobaric hairpin
(ibh), and other
complex paths:
mostly high T,
low P
Clockwise
apparent
paths,
arc-related
Önen and Hall
(2000)
Parkinson (1996)
Maekawa et al.
(1992) (y)
Clockwise
apparent
paths:
subduction –
transform
transition
Ryoke Belt, Japan
(ibh)
Cooma Complex,
Australia (acw)
Mount Isa Inlier,
Queensland,
Australia (acw)
Olary Block,
Australia (acw)
Bunger Hills,
Antarctica (acw)
Later
metamorphism,
Mojave extensional
belt, California,
USA (ibc)
Overprint on
blueschists, Sierra
Nevada, California,
USA
Overprint on
blueschists,
Queensland,
Australia
Sanbagawa Belt,
Japan
Haast Schists,
New Zealand
Mid-crustal levels,
California Coast
Ranges, USA
Bohlen (1987),
Harley (1989)
Yamamoto (1993)
Baba (1998)
Henry and Dokka
(1992)
Sisson et al.
(1989); Sisson and
Pavlis (1993)
Brown (1998a,b)
Johnson and
Vernon (1995)
Rubenach (1992)
Clarke et al. (1987)
Stüwe and Powell
(1989)
Henry and Dokka
(1992)
Hacker (1993)
Little et al. (1992)
Otsuki and Banno
(1990) (z)
Yardley (1982) (z)
Wakabayashi
(1996)
(hypothetical)
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
1994; Christensen et al., 1994; Barton et al., 1994;
Hensen et al., 1995). I will use the term ‘‘actual P –
T path’’ to refer to P –T paths that result from a
single tectonic event. I will refer to P – T paths that
may be the result of multiple tectonic events as
‘‘apparent P – T’’ paths. Depending on the grade of
metamorphism, the closure temperature of geochronologic systems, and the temporal separation of
tectonic events, the distinction between actual and
apparent P – T paths can be blurred. For example, if
the peak metamorphic temperature is high enough,
geochronologic ‘clocks’ may be reset, erasing information on the timing of the pre-peak metamorphism. Multiple tectonic events can occur so
quickly that superimposed metamorphism might be
interpreted as a single event, even if the isotopic
systems were not reset by peak metamorphism. The
southwest Pacific provides good examples of rapid
tectonic transitions. In this region, at least three
separate events of subduction cessation, subduction
polarity reversal, and arc development have occurred within the last 25 million years (Hall,
1996). Some researchers have suggested that the
southwest Pacific is a modern analog of the western
North American Cordillera (e.g., Silver and Smith,
1983; Moores, 1998), so similar complexity of
metamorphic evolution might be expected in the
latter region. Although apparent P – T paths do not
reflect a single tectonothermal event, they still
provide information on the thermal evolution of a
region or package of rocks. Many regions from
which P – T paths have been determined lack sufficiently detailed geochronologic data. Interpretations
of single event tectonic histories derived from such
P – T paths should be viewed with caution.
Notes to Table 1:
a
References listed for anticlockwise P – T paths marked as (‘‘y’’)
do not include calculated P – T paths. I have interpreted anticlockwise type P – T paths from these works based on the mineral
assemblages described therein. For the references marked (‘‘z’’),
Wakabayashi (1996) interpreted the P – T paths as possible
examples of apparent clockwise paths resulting from subductiontransform transition, a different interpretation than offered in the
original papers. Insufficient geochronologic information exists for
many of the P – T paths listed in this table to ascertain whether they
are products of a single tectonometamorphic event or whether they
are a product of multiple events, or apparent P – T paths.
197
3. Stage of observation and P – T path
preservation: the link between earth processes and
the rock record
Many tectonic processes may cause changes in
geothermal gradient and consequent changes in P/T
ratio recorded in metamorphic rocks. Although Earth
processes may produce a variety of P – T paths, the
rock record we observe is governed by preservation of
the P –T path formed by a given process and the stage
in the geologic evolution of a metamorphic terrane
represented by present field exposures (e.g., Wakabayashi, 1996). Although a rock may physically
experience a P –T path, whether such a P –T path is
observed in the rock record depends on several factors
including:
Whether the P –T path survives thermal perturbations associated with exhumation.
The amount of time elapsed since the metamorphic
event.
The different exhumation rates and consequent
time scales characteristic of a given tectonic
setting.
The P/T ratio of the metamorphism.
Mineral reaction kinetics favor recording of counterclockwise P – T paths or isobaric cooling because
the defining parts of such paths occur under conditions of decreasing temperature and reaction rates,
favoring the preservation of earlier formed metamorphic phases. However, exhumation of rocks is commonly associated with events in which the geothermal
gradient increases (slab breakoff, lithospheric delamination, thermal relaxation following thrusting, extension). Exhumation processes may preferentially
produce or preserve clockwise P – T paths while
erasing anticlockwise P –T paths.
The level of exposure may influence what type of
P – T paths are preserved from the same tectonic
setting. The level of exposure is a function of the
amount of exhumation that has occurred and time that
has elapsed since the metamorphic event. As an
example, consider an area where the geothermal
gradient increased and then decreased. In such an
area, shallow to intermediate crustal levels may record
a prograde P – T path or an apparent P – T path.
Retrograde metamorphism may be too feeble to result
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
in recognizable mineralogic changes. This may be
because rocks were exhumed to shallower and cooler
levels and because the geothermal gradient is lower on
the retrograde path. Thus, shallower or intermediate
crustal levels would record the increasing geothermal
gradient or an overprint, a clockwise P – T path. At
deeper crustal levels, the thermal peak may erase the
prograde path, and the retrograde path would occur at
sufficiently high grade to leave a mineralogic record.
Such a retrograde P – T path would reflect decreasing
geothermal gradients and would be an anticlockwise
path.
Different tectonic settings have different time
scales of P –T path preservation and exposure. In
this paper, the terms ‘young’ and ‘old’ will be used
loosely to refer to rocks younger than and older
Fig. 2. Cross-sectional diagrams showing relationships between tectonic processes and generation of P – T paths. Lower case letters and asterisks
correspond to the position of a packet of rock that is represented with the same lower case letters on the P – T path inset for each set of diagrams.
Diagrams C and D show alternative positions c (c1 and c2) for different tectonic scenarios that are also shown on the corresponding P – T
diagram. P – T paths that correspond to each tectonic scenario are schematic and approximate.
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
199
Fig. 2 (continued).
than about 400 Ma, respectively. Subduction and
collisional settings are associated with the highest
exhumation rates, and rates of several mm/year or
more are common (e.g., Ring et al., 1999). Such
settings are most likely to preserve young P – T
paths from deeply buried rocks. P – T paths resulting
from tectonic processes associated with high exhu-
mation rates, or those occurring at shallow crustal
levels, would be preferentially preserved in young
rocks. In contrast, high P metamorphism in tectonic
settings with low exhumation rates may be preferentially exposed in old rocks. Metamorphism associated with processes with high exhumation rates,
and general metamorphism occurring at shallow
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
Fig. 2 (continued).
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
crustal levels would be preferentially eroded away
in older rocks.
The geothermal gradient represented by the type of
metamorphism is also an important factor in the
probability of preservation of a P – T path. Metamorphism at lower P/T ratios (higher than normal geothermal gradients) has a higher probability of preservation
because they record higher grade metamorphism than
the average at a given crustal depth. Conversely, high
P/low T, subduction-related (such as blueschist facies)
metamorphism created by subnormal geothermal gradients has a lower probability of long-term preservation because many tectonic processes can result in
overprinting or erasure of such assemblages.
Another variable that affects the type of P – T path
created and preserved from a given environment is the
location of a packet of rock with respect to dip slip or
oblique faults. For example, in a contractional setting,
a shallowly buried rock may be on the upper plate of a
thrust system, whereas a deeply buried rock may be
on the lower plate. Rocks within the lower plate may
experience greater burial during thrusting, whereas the
rocks within the upper plate of thrust system may
experience exhumation during the same thrusting
event.
4. Some tectonic mechanisms that may result in
P – T paths in the rock record
The mechanisms discussed in the following sections produce perturbations in geothermal gradients
that relax after the event. Thrusting and continuous,
or steady state, subduction depress geothermal gradients, and those gradients increase back to ambient
levels following the cessation of these tectonic processes, resulting in clockwise P – T paths (e.g.,
Oxburgh and Turcotte, 1974; England and Thompson, 1984). In contrast, the other mechanisms discussed raise geothermal gradients, and the gradients
decrease back to ambient levels after the tectonic
event concludes. Extension as a separate tectonic
mechanism is not discussed. Extension commonly
occurs as a part of another tectonic process, such as
extensional collapse of a collisional orogen, synsubduction extensional exhumation (e.g., Platt,
1986; Dewey, 1988), or hot spot-triggered extension
(e.g., Morgan, 1983).
201
4.1. Collisional orogens and thrust faulting
Although the emphasis of this paper is tectonic
mechanisms other than thrusting– heating cycles, I
will briefly review some aspects of thrusting and
collisional tectonics, as they serve as a point of
comparison for the other mechanisms discussed. A
collisional tectonothermal event begins with subduction, then evolves to collision as a continental margin
or other buoyant entity, such as an island arc or
microcontinent, is partly subducted (e.g., Thompson
and England, 1984; Ernst, 1988). The collision
arrests subduction and geothermal gradients, which
had been lowered by the subduction process, increase, resulting in a clockwise P –T path. In the
latter stages of collisional orogenesis, slab detachment or delamination, (e.g., von Blanckenburg and
Davies, 1995; van de Zedde and Wortel, 2001),
convective removal of lithosphere (Platt and England, 1994), and gneiss dome emplacement (e.g.,
Teyssier and Whitney, 2002), can enhance the heating part of the clockwise P – T paths (Fig. 2H).
Exhumation during collisional orogenesis is up to
tens of kilometers and more, and exhumation occurs
by some combination of extensional, erosional, or
extrusional processes (e.g., Platt, 1986; Maruyama et
al., 1996; Ring et al., 1999) (Fig. 2G –J).
Thrust faulting that causes metamorphism is not
restricted to collisional orogens. For example, largescale thrusting and crustal thickening may have taken
place in the Sevier and Laramide belts of North
America. These thrust belts occur in the back arc
region of a continental margin arc-trench system, and
may not have been associated with a collision (e.g.,
Dickinson and Snyder, 1978, but see Maxson and
Tikoff, 1996 for a contrary view). Metamorphic
evolution associated with non-collisional thrusting
may be similar to collisional metamorphism except
that the peak pressures of metamorphism and exhumation rates are lower.
The process of thrust faulting can produce anticlockwise P – T paths in the lower portion of a thick
thrust sheet where initially warmer material is thrust
over cooler material (e.g., Chamberlain and Karabinos, 1987; Spear et al., 1990). Such P – T path
development has been attributed to thrusting from
an area of high geothermal to low geothermal
gradients (e.g., Chamberlain and Karabinos, 1987;
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
Spear et al., 1990). However, even in areas with the
same geothermal gradient, rocks at the base of a
thick thrust sheet should experience cooling when
thrust over other rocks, because such thrusting
should place formerly deeper and warmer material
over cooler, shallower, material. In order for an
anticlockwise path to be generated, the temperature
difference across the thrust fault must be significant.
Although the thrusting process may produce anticlockwise P – T paths, their preservation requires
special circumstances. Anticlockwise P – T paths
formed from such events may be preserved as a
consequence of tectonic transport from a region of
higher to lower geothermal gradient or decreasing
geothermal gradient in the area during the thrusting
event. However, it is difficult to move rocks away
from a heat source with crustal shortening/thrusting.
The decrease in geothermal gradient observed in
many anticlockwise P – T paths may be a consequence of the cooling of the heat source rather than
tectonic transport away from it. Exhumation of the
rocks to relatively shallow levels must take place
while active thrusting occurs beneath them so that
thermal relaxation does not erase the anticlockwise
P –T trajectory.
Examination of most anticlockwise P – T paths
attributed to thrust or collisional settings shows that
their peak metamorphism occurred under high geothermal gradients (e.g., Chamberlain and Karabinos,
1987; Spear et al., 1990). In most collisional settings,
such hot conditions can probably only be achieved
after slab detachment and resulting warming or after
cessation of thrusting and emplacement of melts
related to cessation of thrusting (e.g., von Blanckenburg and Davies, 1995). For anticlockwise P –T path
development, crustal thickening must occur during
cooling after the thermal peak from a delamination
event. Following delamination or convective removal
of lithosphere, crustal thinning is expected instead
(e.g., Platt and England, 1994; von Blanckenburg and
Davies, 1995). Accordingly, it appears that anticlockwise P – T paths should be created during collisional
orogenesis, but such P – T paths are unlikely to be
preserved. An exception may be some types of arc –
continent collision discussed in the section on arcrelated metamorphism.
Clockwise P –T paths from collisional orogenesis
have a high likelihood of preservation and exposure
in young rocks, in part because of the high exhumation rates associated with such settings. Exhumation rates exceeding 1 cm/year and total
exhumation exceeding 100 km have been recorded
in rocks less than 100 My old (e.g., Ring et al.,
1999).
4.2. Inception of subduction
The most extreme anticlockwise P – T paths have
been interpreted from high-temperature metamorphic
rocks progressively overprinted with high-P/low-T
subduction assemblages. Anticlockwise P – T paths
in the high-grade blocks of the Franciscan Complex
of California have been suggested to have resulted
from structural underplating at the inception of
subduction (Wakabayashi, 1990). In this scenario
(Fig. 2A –C), subduction initiated beneath young,
hot oceanic lithosphere, resulting in high-temperature metamorphism on the top of the oceanic crust
of the downgoing plate. Some of this high-grade
metamorphic rock was transferred to the upper plate
(Fig. 2B), either by offscraping/underplating or by
thrust attenuation of a limb of a developing fold in
the oceanic crust at the inception of subduction
(Wakabayashi and Dilek, in press). As subduction
continued, the hanging wall was rapidly refrigerated
and assemblages indicative of a lower geothermal
gradient overprinted the high-T metamorphic slice
(Fig. 2C) (Wakabayashi, 1990). If the upper plate
was imbricated during this process (Fig. 2D), or if
blocks of the metamorphic slice are torn from their
source and dragged to greater depth, additional
burial of the rocks could occur during cooling
(Wakabayashi, 1990). Anticlockwise P – T paths
generated by thermomechanical processes at the
inception of subduction have been simulated by
modeling (Hacker, 1991; Aoya et al., 2002; Gerya
et al., 2002) (Fig. 1A). Structurally, high material in
Cloos’ (1985) subduction model traverses an anticlockwise or isobaric cooling P – T path, although
P – T paths are not discussed in that paper. Subduction initiation results in the creation of metamorphic
soles found beneath ophiolites (e.g., Williams and
Smyth, 1973; Spray, 1984; Jamieson, 1986). Counterclockwise P – T paths have been suggested in
metamorphic soles based on thermal modeling
(Hacker, 1991) and interpreted from metamorphic
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
assemblages in field-based studies (Hacker and
Gnos, 1997; Dilek and Whitney, 1997; Önen and
Hall, 2000) (Figs. 1A and 2A –D).
Anticlockwise P – T paths from the inception of
subduction have a moderate probability of preservation in young rocks. Their probability of preservation is not as high as for clockwise P– T paths from
subduction or collisional settings because of the
extremely small area affected by this type of metamorphism. Anticlockwise P –T paths in subduction
zones generally affect areas of tens of square kilometers or less, and structural thicknesses of a few
hundred meters or less (e.g., Hacker and Gnos,
1997; Dilek and Whitney, 1997; Önen and Hall,
2000). These rocks commonly constitute much less
than 1% of the rocks of a subduction complex (e.g.,
Coleman and Lanphere, 1971; Parkinson, 1996).
Because of the small volume affected by such
anticlockwise paths, preservation of such a record
is unlikely for old rocks, as the rocks recording this
P – T path would probably be completely eroded or
overprinted by a subsequent thermal event. Nonetheless there are examples of this type of metamorphism from numerous orogenic belts (Table 1).
Although they constitute a minute fraction of subduction complex or subophiolitic rocks, these rocks
are important markers of tectonic history because
they record the key signatures of the inception of
subduction.
4.3. Continuous subduction
Continuous subduction, without collision, may
generate clockwise P –T paths in high-P/low-T rocks,
as a result of heat conduction from the cooling
hanging wall of a subduction zone (e.g., Cloos,
1985; Aoya et al., 2002) (Fig. 2C – E –F). Later in
an extended history of subduction the ‘steady state’
P – T may be more of a ‘‘hairpin’’ in which the
exhumation path nearly reverses burial path (e.g.,
Ernst, 1988, 1993; Aoya et al., 2002) (Fig. 1E; 2E –
F). P –T paths resulting from continuous subduction
have a reasonable likelihood of being preserved in
young rocks as long as exhumation occurs while
subduction is active. The cessation of subduction
results in thermal overprinting or erasure of subduction metamorphic assemblages (e.g., Ernst, 1988).
The area affected by each such continuous subduc-
203
tion event can be tens to hundreds of thousand square
km (e.g., Maruyama et al., 1996). Based on their
common occurrence in the world’s orogenic belts
(Aoya et al., 2002; Maruyama et al., 1996; Ernst,
1988), clockwise P –T paths from continuous subduction have a high likelihood of preservation and
exposure in young rocks. Because subduction zone
metamorphism occurs at high P/T ratios (low geothermal gradients), and is associated with high exhumation rates, the likelihood of preservation of
subduction P – T paths is low in old rocks. The rate
of exhumation (and erosion) as well as the probability
of overprinting may be as important (Wakabayashi,
1996) as (possibly) higher Precambrian geothermal
gradients (e.g., Ernst, 1972; Maruyama et al., 1996) in
explaining the scarcity of Precambrian blueschist
terranes (Fig. 2C –F).
4.4. Subduction –transform transition and ridge –
trench interactions: slab window effects
Subduction refrigerates a subduction complex, creating the lowest geothermal gradients on Earth (e.g.,
Ernst, 1988). The cessation of subduction will result
in warming, and development of clockwise P– T paths
(e.g., Ernst, 1988). Collisions of buoyant terranes,
such as volcanic arcs, continental margins or microcontinental fragments, are the most commonly proposed mechanisms by which subduction is arrested
(e.g., Ernst, 1988) (Figs. 2K – L, 3 and 4).
Collisions are not the only mechanism that can
result in the heating of a subduction complex, nor are
they the only mechanism that halts subduction. Heating of a subduction complex can take place by
subduction of an oceanic spreading ridge, called a
ridge –trench interaction, with or without cessation of
subduction (Sisson and Pavlis, 1993; Underwood et
al., 1993; Brown, 1998a). Such heating can also occur
as a result of conversion of subduction zone to a
transform plate boundary by migration of a fault –
fault – trench triple junction (Furlong, 1984), called a
transform – trench interaction. I will refer to ridge –
trench and transform –trench interactions collectively
as triple-junction interactions and the corresponding
metamorphism as triple-junction metamorphism. Triple-junction interactions involve the creation of slabfree windows through which asthenospheric upwelling occurs, resulting in heating and magmatic under-
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
plating (e.g., Dickinson and Snyder, 1979; Furlong,
1984; Thorkelson, 1996). Igneous activity is common with triple-junction interactions, resulting in
eruption of volcanic rocks and emplacement of
calc-alkaline plutons (e.g., Liu and Furlong, 1992;
Dalrymple et al., 1999). If such a terrane is deeply
exhumed, the field relations may appear arc-like
(Wakabayashi, 1996; Brown, 1998a).
The distribution of subduction complex rocks
affected by ridge –trench and transform –trench interactions is determined by the angle between the ridge
crest and the trench and the obliquity of subduction.
For ridge strikes less than 90j clockwise of the
subduction zone strike in Fig. 3 (inset and A to D),
both ridge – trench and fault –trench interactions occur. The slab windows associated with the resulting
ridge – trench and transform –trench interaction pairs
have been termed ‘‘fraternal’’ slab windows (Thorkelson, 1996). An example is coastal California,
where the Rivera triple junction, a ridge –fault – trench
triple junction, has migrated southward and the Mendocino triple junction, a fault – fault – trench triple
junction has migrated northward, as a lengthening
transform boundary (the San Andreas fault system)
replaces the former subduction boundary between the
two triple junctions (e.g., Atwater and Stock, 1998).
Subduction of the ridge – transform system will result
in the subduction complex being affected by either a
ridge – trench or a transform – trench metamorphic
event (Fig. 3C). For ridge strikes of 90j or more
clockwise of the trench strike in Fig. 3 (inset and E to
G), only ridge –trench interactions occur. In this case,
a subduction zone may experience a succession of
ridge – trench events separated by episodes of normal
subduction. An example of this geometry is the Chile
triple-junction area and affected reaches of the subduction zone (e.g., Forsythe and Nelson, 1985;
Lagabrielle et al., 2000). The azimuth of subduction
will also influence the relative lengths of trench
affected by different types of triple-junction interactions. Regardless of the obliquity of subduction,
transform – trench type interactions can only occur
for half of the range of possible angles between the
ridge and trench (Fig. 3 inset). There are also subduction azimuths for which ridge – trench interactions
will effect the entire length of a trench, even for ridge
strikes favorable to ‘‘fraternal slab window’’ development. For example, for Fig. 3A – D, imagine a case
in which the subduction azimuth is either parallel to
the oceanic transform or subduction that has a large
dextral component. In the former case, there will be
negligible length of trench affected by transform –
trench interaction. In the latter case, trench segments
affected by ridge – trench interactions overlap and
segments with transform – trench interaction will be
overprinted. The geometries of triple-junction interactions and effects indicate that ridge – trench interactions should affect greater lengths of former
trenches than transform – trench interactions.
Triple-junction interactions result in rapid heating,
commonly occurring in 5 Ma or less, to a thermal
peak, followed by slower cooling (e.g., Furlong,
1984). In the case of coastal California, there has
been less than 3 km of exhumation in most areas in
the 25 My since triple-junction interactions began
Dumitru (1989). This suggests that there has been
little vertical crustal movement associated with the
heating and cooling following that particular triplejunction interaction (Wakabayashi, 1996). The
resulting P – T path may involve nearly isobaric
heating and cooling, essentially an isobaric hairpin
P – T path (Wakabayashi, 1996) (Fig. 2K – L). The
Chugach terrane in Alaska and the Ryoke Belt of
Japan record isobaric hairpin P –T paths, and their
metamorphism has been attributed to ridge –trench
interaction (Sisson et al., 1989; Sisson and Pavlis,
1993; Brown, 1998a,b). If peak and subsequent
metamorphism erases prograde assemblages, the
resultant P – T path for triple-junction metamorphism should feature isobaric cooling. Higher P/T
ratios may be associated with transform – trench
metamorphism than ridge – trench metamorphism,
because the heat source is deeper in the former
case (compare Fig. 1D to paths S89 and Br98 of
Fig. 1B).
Where sufficient metamorphic temperatures are
reached and sustained during cooling during a triplejunction metamorphism event, earlier, subduction-related, metamorphism may be completely erased.
However, at shallower crustal levels, where triplejunction metamorphic temperatures are lower, subduction-related metamorphism may be preserved.
high-P/low-T subduction-related rocks exhumed to
relatively shallow levels during subduction may not
leave a mineralogic record of the exhumation path
because the exhumation took place under low geo-
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
205
Fig. 3. Diagrams illustrating triple-junction interactions along trenches. No absolute scale is implied; ridge segments may range from 10 to 100 s
of km in length. The interaction of a ridge – transform system with a trench can result in either a series of ridge interaction events, or a ridge –
trench interaction paired with a fault – fault – trench interaction (see inset). Diagrams A to D show a sequence of subduction of spreading/oceanic
transform system based on the interaction of fraternal slab windows (terminology of Thorkelson, 1996). With complete subduction of the
spreading system, the margin may be converted to a transform margin, and shifting of the location of strike-slip faults may move higher P/T
fault – fault – trench-influenced rocks outboard of ridge – trench-influenced rocks (D). This produces a paired metamorphic belt configuration
(similar to concept proposed by Brown (1998a)). Diagrams E to G illustrate the formation of paired belts with ridge – trench interactions only,
the other half of the angular possibilities. Such interactions can result in a subduction complex unaffected by slab window metamorphism being
juxtaposed with an inboard low P, high T metamorphic belt that was affected by ridge – trench interaction.
thermal gradients and low temperatures (Ernst, 1988).
At shallow crustal levels, partially exhumed subduction complex rocks affected by triple-junction metamorphism may develop an apparent P – T path in
which greenschist facies metamorphism overprints
blueschist facies metamorphism (Figs. 1H and 2E –
F, K – L) (Wakabayashi, 1996). The apparent P – T
path developed in the sequence of diagrams Fig.
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
2E –F, and Fig. 2K – L illustrates a hypothetical P –T
path developed in Franciscan Complex blueschist
facies rocks that are still several kilometers beneath
the surface of the present California Coast Ranges
(Wakabayashi, 1996). These rocks were originally
metamorphosed under blueschist facies conditions in
the Cretaceous, then exhumed to mid-crustal levels
while subduction was still active. No significant
metamorphic mineral growth developed during the
cold exhumation of the rocks. Following the partial
exhumation, the triple-junction metamorphism occurred and the partly exhumed blueschist facies rocks
were overprinted with greenschist facies assemblages
creating an apparent clockwise P – T path. With many
geochronologic methods, it may be difficult to determine whether such overprinting mineral relationships
are a consequence of a single event, or an apparent P –
T path. In many blueschist terranes, the only datable
metamorphic mineral is white mica (e.g., Maruyama
et al., 1996) and the closure temperature for Ar in
white mica may be lower than the temperature of the
greenschist faces overprint (e.g., Grove and Bebout,
1995). Greenschist facies triple-junction-related metamorphic overprints may completely reset white mica
ages, leaving no geochronologic record of the earlier
blueschist metamorphism.
Triple-junction metamorphism may be associated
with fabrics that reflect orogen-parallel extension and
shear related to the development of a transform plate
boundary (Wakabayashi, 1996; Brown, 1998a). Overprints on blueschist facies rocks are commonly associated with orogen-parallel stretching lineations,
suggesting that many such overprints may be caused
by triple-junction interaction, rather than subduction –collision, as previously believed (Wakabayashi,
1996). The deepest levels of triple-junction metamorphism may reach granulite facies (Furlong, 1984;
Sisson and Pavlis, 1993; Wakabayashi, 1996; Maeda
and Kagami, 1996; Brown, 1998a). Transform –
trench interactions has been proposed as a mechanism of formation for some medium to high-P (6 –10
kbar) granulites (Wakabayashi, 1996), and ridge –
trench interactions has been proposed for the development of low-P (~4 kbar) granulite belts, and lowP/high-T metamorphism in general (Brown, 1998a).
Newton (1990) noted that a transition from thrusting
to strike-slip deformation characterizes many medium
to high-P granulite belts. Such a sequence of defor-
mation is consistent with the hypothetical structural
evolution during subduction – transform transition.
The relationship between CO2-rich fluids and granulite formation has been suggested (Newton, 1989).
These fluids are thought to be mantle derived based
on mass balance considerations (Newton, 1989) and
carbon isotopic studies (Jackson et al., 1988). Triplejunction metamorphism and associated slab window
development involves direct contact between mantle
(upwelling asthenosphere) and lower crust. Along the
San Andreas fault system, strike-slip faults associated
with the transform fault plate boundary cut through
the entire crust, based on recent seismic studies
(Parsons et al., 2002). These faults provide potential
pathways for mantle-derived fluids to reach the lower
crust after magmatic underplating associated with
slab window development occurs. Thus, triple-junction metamorphism and subsequent transform tectonics can provide an environment for an extended
history of interaction between mantle-derived fluids
and the lower crust.
Depending on the timing of transpressive or transtensional deformation, resulting from minor changes
in relative plate motion or irregularities in strike-slip
fault geometries (step-overs and bends), small pressure increases or decreases during heating and cooling may occur. Fluctuations in burial depth related to
transpression or transtension may be as large as 10
km along the San Andreas fault system, based on the
stratal thickness of strike-slip basins, and the structural relief generated by fold and thrust belts inverting such basins, (e.g., Yeats, 1987). Superposition of
such fluctuations in burial depth will influence of the
P – T path generated by triple-junction metamorphism. For example, the triple-junction interaction
event may be associated with transpression and
crustal thickening, followed by an extended period
of nearly pure strike-slip. Such a history may result
in crustal thickening during heating, with increasing
burial for deeper rocks, followed by near isobaric
cooling (e.g.,Williams and Karlstrom, 1996). Conversely, initial ridge – trench interaction may be associated with nearly pure strike-slip followed by
transpression. Such a P – T path may feature near
isobaric heating, followed by an anticlockwise segment of a path, perhaps resulting in an anticlockwise
P – T path similar to the JV95, Rb92 paths in Fig. 1B.
During the history of a transform margin, the relative
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
plate motion may shift between transtensional and
transpressional and visa versa. For example, in the
San Andreas transform system, plate motion across
the system may have been transtensional from the
inception of the transform fault system at 18 to about
8 Ma, after which relative plate motion became
transpressional (Argus and Gordon, 2001). Oscillations between transpression and transtension along a
transform plate boundary may cause alternating burial and exhumation, that may result in complex P – T
paths for triple-junction interactions. The ‘‘yo yo
tectonics’’ recorded by geothermobarometry in metamorphic rocks along a basement wrench zone in
Anatolia (Casale et al., 2002) may be an example
of the metamorphic signature of this type of tectonic
mechanism.
An interesting consequence of triple-junction
interactions is the potential to juxtapose belts of
contrasting metamorphic styles and P – T evolution
across strike-slip faults (Brown, 1998a). Brown
(1998a,b) concluded that many paired metamorphic
belts, particularly the Sanbagawa and Ryoke belts of
Japan, became juxtaposed as a result of triple-junction interaction and subsequent strike-slip faulting,
rather than being a product of paired subduction and
arc-related metamorphism (e.g., Miyashiro, 1967,
1973). Possible mechanisms for such pairing of
metamorphic belts are shown in Fig. 3, which differs
from the diagrams of Brown (1998a) in showing
both ridge – trench interactions and transform – trench
interactions. In the mechanisms shown in Fig. 3,
strike-slip faulting moves parts of the subduction
complex outboard of those that have experienced
higher temperature triple-junction metamorphism.
Paired metamorphic belts can also be formed by
ridge – trench interaction without strike-slip faulting
(Fig. 4). Ridge –trench collision may result in the
stalling of the oceanward side of the spreading
center at the trench. The stalling of the oceanic
crust may occur because there is no downgoing slab
to pull on the ocean crust on the oceanward side of
the spreading center and the leading edge of that
section of ocean crust is very young and buoyant. If
subduction resumes, a new subduction zone may
form seaward, leaving a trapped piece of oceanic
crust (ophiolite). Examples of such ophiolites are
found on the Taitao Peninsula of Chile and in
southern Alaska (Forsythe and Nelson, 1985; Lyt-
207
wyn et al., 1997). As subduction continues geothermal gradients will decrease again, and a subduction
complex will form with typical subduction zone
metamorphism, outboard of the older subduction
complex that was affected by high-T/low-P ridge –
trench metamorphism. This tectonic mechanism may
produce paired metamorphic belts, but the highgrade metamorphism in the inboard belt should be
slightly older than the oldest high-P/low-T metamorphism in the outboard belt.
For ridge –trench metamorphism, the probability of
exposure in young rocks is probably high because the
site of metamorphism is along an active plate margin,
the depth of the highest grade levels of metamorphism
is not excessive, and the metamorphism occurs at high
geothermal gradients. Brown (1998a) suggested that
ridge subduction may be the general process by which
low-T, high-P terranes are formed. Such terranes
occupy areas of up to tens of thousands of square
kilometers in both Phanerozoic and Precambrian orogenic belts, although they are more common in the
latter (Brown, 1998a). Nelson and Forsythe (1989)
suggested that ridge –trench interactions were a major
process in Archean crustal growth. Implicit in their
suggestion is the premise that such interactions
strongly influenced metamorphism in Archean orogenic belts.
For transform – trench interactions, the probability
of exposure of amphibolite and granulite grade rocks
in younger orogenic belts is lower than that of ridge –
trench interactions because the metamorphism occurs
deeper. The rates of exhumation along transform
margins are probably insufficient to exhume the
deeper (20 km or more) levels within a short period
of time. For example, exhumation rates along most of
the San Andreas transform plate boundary are less
than 0.2 mm/year, averaged over 20 million years or
more of transform plate boundary history (Dumitru,
1989). Medium-P granulites generated by this process
would likely be found predominantly in older rocks
(Wakabayashi, 1996). This premise that medium to
high-P granulite terranes are associated with environments of low long-term exhumation rates is consistent
with the observations of Newton (1987), who showed
the increasing area of granulite terranes with increasing age. The shallower levels of transform – trench
metamorphism are more likely to exposed within
younger rocks, and this process may account for the
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
thermal overprint on some high-P/low-T terranes
(Wakabayashi, 1996).
The terminal event in the closure of an ocean basin
is continent – continent collision, and such events form
the most conspicuous orogenic belts on Earth (e.g.,
Dewey and Bird, 1970). Unless spreading ceased long
before the terminal collisions, triple-junction interactions should have affected significant lengths of the
active plate margins that eventually became parts of
the collisional orogenic belt. I suggest that many
collisional orogens may have earlier metamorphic
records of pre-collisional triple-junction metamorphism. Triple-junction interactions are common on
the present-day Earth, so such processes were probably equally common in the geologic past and have left
a significant record in orogenic belts (Sisson et al.,
1994).
4.5. Initiation and cessation of arc magmatism
Magmatic arcs have long been suggested as an
environment of elevated geothermal gradients and
associated metamorphism (e.g., Miyashiro, 1967,
1973). Geothermal gradients should increase in an
area when arc magmatism begins and decreases after
the shut off of magmatism. Initiation and cessation of
arc magmatism in a region may result from starting
and stopping subduction, but can also result from
across-strike migration of arc volcanism associated
with trench rollback or shallowing of subduction. Arc
environments may have significant vertical tectonism
associated with them, although this amount is smaller
than that associated with subduction or collisional
settings (e.g., Renne et al., 1993). Both transpressional
and transtensional deformation may occur within an
active arc (Tobisch et al., 1995). In the late Jurassic to
late Cretaceous Sierra Nevada arc of California,
exhumation during the late stages of arc development
of 5 to 10 km was common, and greater amounts may
have occurred locally (Ague and Brimhall, 1988;
Renne et al., 1993; Tobisch et al., 1995; Pickett and
Saleeby, 1993). In the southernmost Sierra Nevada,
100 –115 Ma plutons emplaced at depths of about 30
209
km were fully exhumed before 52 Ma, some 30 Ma
after shut off of the magmatic arc (Pickett and Saleeby, 1993) (Fig. 2M –O).
Associated with the lower levels of magmatic arcs
is another possible cause of burial depth ( P) increase,
magmatic loading (e.g., Brown and Walker, 1993;
Brown, 1996). In such a mechanism, the intrusion
of sheets of magma will result in an increase of
loading of material below. Magmatic loading should
result in an increase in P and T for the deeper parts of
the arc, even without thrust faulting (Brown and
Walker, 1993; Brown, 1996).
In the Sierra Nevada, exhumation of arc rocks
occurred during arc activity and was complete within
the first 20 Ma or so after magmatism ceased, based
on the presence of Eocene overlap assemblages and a
dramatic decrease in forearc basin sedimentation rates
after the Paleocene (Wakabayashi and Sawyer, 2001).
Exhumation coinciding with the last stages of magmatism and the first 20 Ma after cessation of magmatism was generally 10 km or less (Ague and Brimhall,
1988). From the Eocene until about 5 million years
ago, exhumation of the Sierra Nevada was no more
than about 200 m (Wakabayashi and Sawyer, 2001).
If the Sierra Nevada is representative of magmatic
arcs, then the tectonic history can be used to infer
metamorphic P –T paths for such environments. The
type of P – T path preserved from arc initiation and
shut off may be a function of the crustal level exposed
as well as the amount of vertical tectonism associated
with arc development and shut off. At shallower
levels, pre-arc metamorphism may be preserved, with
a low-grade arc-related overprint forming an apparent
P– T path. Retrograde metamorphism after arc shutoff would occur under conditions of lower geothermal
gradient and shallower crustal levels as exhumation
proceeds, favoring recording of an apparent prograde
path over the retrograde path. Such an apparent P – T
path would be noticeable only on partly exhumed
high-P/low-T (blueschist facies) assemblages, otherwise the arc-related overprint would be difficult to
distinguish from older retrograde metamorphism of
higher grade rocks. Such an apparent P –T path should
Fig. 4. Cross-sectional diagrams showing how the interaction of a spreading ridge with a subduction zone and renewed subduction can form
parallel belts of high-P/low-T and low-P/high-T rocks. If significant exhumation occurs, low-P/high-T rocks will be exposed in the inboard belt,
whereas high-P/low-T rocks will be exposed in the outboard belt. The high-grade metamorphism in the inboard belt should be slightly older than
the oldest high-P/low-T metamorphism in the outboard belt.
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J. Wakabayashi / Tectonophysics 392 (2004) 193–218
be a clockwise path, reflecting the increase in geothermal gradient represented by the arc over the prearc environment (Fig. 2E – F, M– O). An arc-related
metamorphic overprint and resulting apparent P –T
path may occur in the northern Sierra Nevada. In this
region, metamorphism which may have resulted from
increased geothermal gradients associated late Jurassic arc magmatism, overprinted blueschist facies rocks
from an earlier, unrelated episode of subduction
(Hacker, 1993). Another arc-related overprint occurs
in blueschist facies rocks in Queensland, Australia,
where overprinting may have occurred as arc magmatism migrated into the former subduction complex as a
result of trench rollback (Little et al., 1992).
At deep enough crustal levels, pre-arc assemblages
may be completely replaced, so only metamorphism
related to arc development will be preserved. Crustal
thickening associated with syn-magmatic transpressional deformation may have been associated with
increases in P and T for deeper rocks and exhumation
for shallower rocks (Fig. 2N – O). The 50 Ma period
with negligible exhumation that followed arc shut off
in the Sierra Nevada may be associated with isobaric
cooling. The combined history of heating and cooling
for the deeper levels of Sierra Nevada arc-related
metamorphism may have resulted in an anticlockwise
P –T path followed by isobaric cooling, if the rocks
were on the footwall of transpressional fault systems
(Fig. 2N – O). For the deepest and highest grade rocks
beneath the Sierra Nevada, the prograde path may be
erased leaving only the isobaric or anticlockwise
cooling path. The peak P and T conditions of 8 kbar
arc-metamorphic rocks in the southern Sierra have
been estimated, but P – T paths have not be calculated
(Pickett and Saleeby, 1993). Peridotite xenoliths erupted from late Cenozoic volcanoes and derived from
the mantle from beneath the Sierra Nevada batholith
record isobaric or nearly isobaric cooling, interpreted
to be of Cretaceous age (Lee et al., 2000). The
isobaric cooling recorded in the xenoliths may reflect
cooling after shut off of the arc. Deeply buried rocks
from arcs associated with less syn-magmatic vertical
tectonism than occurred during Sierra Nevada history
may feature near isobaric hairpin P – T paths.
The base of volcanic arcs has been suggested as a
likely environment for granulite formation (Bohlen,
1987), and isobaric or near isobaric cooling after
cessation of arc magmatism is consistent with P –T
paths associated with many granulite belts (Bohlen,
1987). However, high-grade, ~8 kbar metamorphic
rocks exposed in the southern Sierra Nevada were
associated with metamorphic fluids that had a higher
water activity than fluids associated with typical
granulite formation (Pickett and Saleeby, 1993). In
contrast, arc-metamorphic rocks from the Coastal
Cordillera or northern Chile are 5 kbar granulites that
are overprinted by amphibolite facies assemblages
(Lucassen and Franz, 1996). These rocks show near
isobaric cooling from granulite to greenschist facies.
The 8-kbar plutons in the southern Sierra are granitic
(Pickett and Saleeby, 1993), whereas the 5-kbar metamorphic rocks from northern Chile are metabasaltic
and interpreted to be the remnants of basaltic magmas
underplated at the base of an arc (Lucassen and Franz,
1996). The contrast between the two regions suggests
a significant difference in the thickness of the batholithic rocks, differences in geothermal gradients, and
possibly differences in the metamorphic mechanisms.
Some of these differences may be because the Chilean
rocks were associated with an oceanic arc (Buchelt
and Cancino, 1988; Vergara et al., 1995), whereas the
Sierra Nevada arc was a continental margin arc (e.g.,
Ague and Brimhall, 1988). The Kohistan arc of
Pakistan, however, with much higher P metamorphism (>10 kbar), is interpreted as an oceanic arc
(e.g., Bard, 1983). An alternative hypothesis for the
origin of the Chilean high-grade rocks is that they are
the product of triple-junction metamorphism or delamination-related metamorphism instead of arc-related metamorphism.
Based on the above discussion, it appears that
some variety of metamorphic P – T paths can be
expected from the deeper rocks associated with magmatic arc metamorphism. In addition to the P –T paths
discussed above, anticlockwise P –T paths associated
with large changes in P have been interpreted from the
base of the Kohistan Arc in Pakistan (Yamamoto,
1993). Such P –T paths may result if the arc is on the
lower plate of a collision zone. For such anticlockwise
P – T paths to form, arc magmatism should be extinguished prior to or during this partial subduction. The
arc shut off itself may be associated with another
collision, or a subduction – transform transition, that
stopped the subduction associated with the development of that arc. Similar, collisionally related contraction during cooling of an arc may also explain the
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
pronounced anticlockwise P – T evolution of some
high P/high-T terranes such as those shown in the
oldest part of path HD92 and in path B98 in Fig. 1B.
This type of tectonic evolution represents a special
case of arc-related metamorphism, but such collisions
may be relatively common in the geologic record,
because several events of this nature have occurred
during the Cenozoic in the southwest Pacific (Hall,
1996). Similar collisional events of this type have
been proposed for the Phanerozoic development of the
North American Cordillera (e.g., Moores, 1970).
Exhumation rates after shut off of magmatic arcs
may be low, whereas syn-magmatic exhumation rates
may be much higher, if the Sierra Nevada example is
representative. Accordingly, exposures of arc-metamorphic rocks that record arc shut off may be relatively rare in young rocks, whereas arc-metamorphic
rocks lacking the arc shut off signature may be
relatively common. Examples of moderately deep
arc rocks and their metamorphic products are common
in young rocks from the North and South American
Cordillera (Zen, 1988; Pickett and Saleeby, 1993;
Lucassen and Franz, 1996). Exposure of the deep
roots of Mesozoic magmatic arcs also occur, including
the Kohistan Arc in Pakistan (e.g., Bard, 1983), and
the Fjordland region in New Zealand (e.g., Bradshaw,
1989; Brown, 1996; Clarke et al., 2000).
Active magmatic arcs cover large areas of the
Earth. Given the common occurrence of magmatic
arcs throughout Earth history, it is likely that P – T
paths resulting from arc magmatism have left a record
in many orogenic belts.
4.6. Subcontinental delamination (distinct from slab
delamination during subduction – collision)
In some areas, the mafic lower continental crust
and subcontinental lithosphere may founder (e.g.,
Furlong and Fountain, 1986). The delamination considered in this section is distinct from the slab
detachment that may occur in subduction – collision
orogens. Delamination may result in asthenospheric
upwelling that will raise the geothermal gradient
above a normal continental geotherm, followed by
possible magmatic underplating, and thermal relaxation (cooling) back to a normal geotherm (Furlong
and Fountain, 1986). Asthenospheric upwelling and
magmatic underplating are associated with eruption
211
of potassic felsic volcanic rocks, some alkali basalts,
and emplacement of granite plutons (e.g., Moore and
Dodge, 1980; Ducea and Saleeby, 1998a,b; Manley et
al., 2000). Surface and rock uplift may accompany
the delamination event, as a result of the lithospheric
buoyancy created by the delamination of dense material (Ducea and Saleeby, 1998b). The amount of
exhumation directly associated with such an event
may be relatively small. For example, the eastern
Sierra Nevada region, under which delamination may
have occurred within the last 8 Ma (Ducea and
Saleeby, 1998b), has experienced a maximum of
about 1 km of exhumation within the past 4 to 5
Ma (Wakabayashi and Sawyer, 2001). As underplated
magmas crystallize and the heated area cools, exhumation may slow down or cease (e.g., Crough, 1983)
(Fig. 2P – R).
The metamorphic signature of a delamination event
likely depends on the level of exposure. At upper
crustal levels, pre-delamination assemblages may be
preserved, and if those assemblages are of sufficiently
low grade, an apparent P – T path may be formed by
the superposition of peak delamination-related metamorphism on earlier metamorphic assemblages. At
deeper crustal levels, pre-existing assemblages may be
erased and a near isobaric hairpin P – T path is
expected, because of the comparative lack of delamination-related exhumation. A near isobaric cooling
path is expected if prograde assemblages are not
preserved. The tectonic setting of subcontinental delamination and associated magmatic underplating has
been proposed as an environment of granulite formation, and near isobaric cooling paths found in some
granulites are consistent with this type of environment
(Bohlen, 1987). Delamination also provides a means
of delivering mantle-derived fluids into contact with
the lower crust. Along the eastern margin of the Sierra
Nevada, dextral and normal faulting was associated
with or coincident with delamination, and has continued to the present day (e.g., Wakabayashi and Sawyer,
2001). This faulting may cut the entire crust, because
alkali basaltic rocks erupted along these faults are
thought to be mantle derived (e.g., Moore and Dodge,
1980; Ormerod et al., 1991). This faulting provides
pathways for mantle-derived fluids to continue to
drive metamorphism in the deep crust after the initial
delamination and underplating event. The relationship
between strike-slip faulting, mantle-derived fluids,
212
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
and high-grade metamorphism has been noted in a
number of granulite terranes (e.g., Newton, 1989,
1990). In the case of the Sierra Nevada, deep metamorphism related to delamination has occurred or will
occur in rock affected by earlier arc-related metamorphism. Differentiating between delamination and arc
metamorphism may be difficult in the rock record. If
the tectonic history of a plate margin is well enough
known, the two environments may be distinguished on
the basis of age of metamorphism compared to the
timing of subduction and arc magmatism. In the Sierra
Nevada case, delamination-related metamorphism
should be 80 Ma younger than arc-related metamorphism (e.g., Ducea and Saleeby, 1998a) and it occurred
at a time when the plate boundary is a transform one.
Distinction between this type of delamination and a
triple-junction-related metamorphism can be made on
paleogeographic relationships. In late Cenozoic California, triple-junction interaction has overlapped in
time with delamination, but the triple-junction interaction affects former subduction complex rocks at
the continental margin, whereas the delamination
occurs over 100 km inboard of the old subduction
complex.
The area affected by subcontinental delamination
events appears to be on a scale comparable to the size
of volcanic arcs, if the Sierra Nevada example is
representative (Ducea and Saleeby, 1998a,b). If we
consider only delamination events that are distinct
from other types of tectonic events, the amount of
exhumation associated with such events appears to be
relatively small. Consequently, the probability of
exposure of the deep levels of such thermal events
in young rocks is low. Relict assemblages overprinted
by shallow levels of subcontinental delamination
metamorphism are less likely than arc-related metamorphism to include high-P/low-T rocks, the only
rocks in which the delamination overprint would be
noticeable. Such rocks would be less likely to be
associated with subcontinental delamination than
magmatic arc initiation, because the former process
is not restricted to occur near a continental margin as
the latter is. Thus, subcontinental delamination events
(as defined herein) may have had little noticeable
impact on metamorphic P – T paths in shallowly
buried young rocks. In contrast, the deeper, highergrade rocks produced by delamination events may be
an important component of old orogenic belts.
4.7. Approach and departure of a hot spot
Hot spots are known for their association with
major igneous activity, particularly the eruption of
flood basalts (e.g., Morgan, 1983), so they must have
a significant metamorphic impact as well. Many hot
spot tracks cross continents, so it is likely that they
have had a major influence on the tectonic and thermal
history of some orogenic belts (e.g., Morgan, 1983;
Murphy et al., 1998). Geothermal gradients increase as
the hotspot approaches (e.g., Nathenson and Guffanti,
1988), and significant exhumation and uplift may
occur (e.g., Crough, 1983; Parsons et al., 1994; Murphy et al., 1998). Cooling and a return to pre-hot spot
tectonics and thermal regimes are expected as the hot
spot moves away from an affected area (e.g., Anders
and Sleep, 1992). The Yellowstone hotspot, that has
crossed part of western North America in the Cenozoic, is a good example of the impact of a hot spot on a
continental margin (Anders and Sleep, 1992; Parsons
et al., 1994; Murphy et al., 1998; Humphreys et al.,
2000). The hotspot has either triggered or influenced
extensional tectonics in the Basin and Range province
(Morgan, 1983; Parsons et al., 1994; Murphy et al.,
1998). Although the peak metamorphism associated
with hot spot migration are superficially similar to that
produced in other environments of high-grade metamorphism, there appear to be some significant differences. Hot spots commonly migrate across the
structural grain of a continental margin, whereas
delamination, triple-junction interactions, and magmatic arcs produce metamorphic belts parallel to
subparallel the structural grain (e.g., Morgan, 1983;
Keppie et al., 2003) (Fig. 2S – U).
The effects of a hot spot interaction are superimposed on the existing tectonic regime (e.g., Anders
and Sleep, 1992; Parsons et al., 1994; Murphy et al.,
1998, 1999). Hot spots are involved with prolonged
heating with a continued delivery of hot material from
an ascending plume, whereas delamination is usually
followed by a single upwelling event and cooling (e.g.,
Duncan and Richards, 1991). The prolonged heating
results in the eruption of greater volumes of primitive
mafic lavas, including eruption of flood basalts and
emplacement of mafic intrusives, (e.g., Duncan and
Richards, 1991; Anders and Sleep, 1992), and greater
exhumation as a result of extensional tectonics (Parsons et al., 1994), compared to delamination events.
J. Wakabayashi / Tectonophysics 392 (2004) 193–218
The type of P – T path preserved is in part a
function of the level of crustal exposure. At shallow
crustal levels, preservation of pre-existing metamorphic assemblages may lead to the development of
apparent P– T paths, that may be noticed as such only
if the pre-existing rocks are high-P/low-T rocks.
Deeper levels of hot spot metamorphism will likely
erase preexisting assemblages. Because extension and
exhumation associated with hotspot metamorphism
may be significant, pressure may decrease during
heating leading to a clockwise P – T path. A clockwise
P – T path has been determined for low-P/high-T
Devonian metamorphism in the northern Appalachians (Keppie and Dallmeyer, 1995) that has been
attributed to heating by a hot spot (Murphy et al.,
1999; Keppie and Krogh, 1999). Keppie et al. (2003)
suggested that hot spot interaction resulted in a
clockwise P – T path for Jurassic low-P/high-T metamorphism and related deformation in the Acatlan
Complex of Mexico. Extension and exhumation slow
down after the passage of the hotspot (Anders and
Sleep, 1992), so the last part of the retrograde metamorphic P –T path may approximate isobaric cooling.
Hot spot metamorphism has been suggested as one
mechanism of forming granulites (Bohlen, 1987;
Newton, 1987). Morgan (1983) suggested that hot
spots traversing a continent may form zones of
weakness and trigger continental rifting. The timing
of the passage of some hot spot tracks over the
continents suggests that their passage may coincide
with high-temperature metamorphism in certain
regions. For example, the Bermuda hotspot may have
passed through the area of the Sevier thrust belt at or a
bit before 120 Ma (Morgan, 1983). The passage of the
hotspot may have influenced both the metamorphism
and the deformation in that area. The Bermuda hot
spot may have also contributed to exhumation in the
central and southern United States (Crough, 1983).
Crough (1983) also suggested that some Jurassic
intrusions in New Hampshire occurred as a result of
interaction with the Cape Verde hot spot.
The significant exhumation associated with hot spot
interactions suggests a reasonably high probability of
exposure in young rocks for the deeper levels of the
crust affected by hot spot metamorphism. The heating
from a hot spot can affect an area of tens of thousands
to hundreds of thousands of square kilometers (e.g.,
Crough, 1983; Sleep, 1990; Humphreys et al., 2000). It
213
is likely that hot spot interactions have left a much
larger impact on the metamorphic record than has been
previously appreciated (Murphy et al., 1998).
5. Conclusions
This paper has speculated on several tectonic
mechanisms that result in metamorphic P – T paths
in regionally metamorphosed rocks. The tectonic
mechanisms described herein certainly do not cover
all possible causes of P –T path development. However, the tectonic processes discussed serve to illustrate that there are probably many more causes of
metamorphic P – T paths than are commonly considered in analyses of orogenic belts. In this paper, I have
treated each type of tectonic interaction singly. During
the evolution of an orogenic belt, many of these
mechanisms may operate, interact with, and overprint
one another on relatively short time scales. Although I
have attempted to add greater complexity and flexibility to tectonic interpretations of metamorphic P– T
paths, this effort itself is probably a gross simplification of actual tectonothermal histories experienced by
many orogenic belts.
Acknowledgements
I am indebted to many for thoughtful discussions
on the relationship of metamorphism and tectonics.
Above all I acknowledge W.G. Ernst, whose studies
on the subject have served as continuing motivation
and inspiration. I thank J. Selverstone and T. Kato for
the helpful, constructive reviews.
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